In general, sensors are based on the principle that changes of electrical parameters of a device by an external influence, such as the measured quantity to be sensed, are sensed and evaluated. Thus, in a capacitive pressure sensor, the capacitance of a capacitor element changes when a capacitor plate of the sensor element formed as a membrane is deflected as a result of changing ambient pressure. A measurement circuit accordingly measures the change of the electrical parameters as a capacitance change of the capacitor and converts this electrical parameter into an analog output signal or a digital value. The output signal or digital value thus obtained are then transferred to a processing means via a signal path and evaluated by the same, in order to finally obtain an indication of the measured quantity to be sensed.
A conventional technique for reading out capacitive sensors (sensor capacitances) consists in the so-called switched capacitor (SC) technique, for example. Here, a reference voltage is sampled with a sensor capacitance, and the change in charge proportional to the capacitance change or the resulting current flow is processed further. Since the resulting sensor signal usually is to be digitized, a so-called delta/sigma modulator is frequently used as a further processing circuit.
A disadvantage of the known switched capacitor technique consists in the sampling of the white noise in the sensor output signal, which develops, for example, when reading out capacitive sensors by the so-called ON resistances (turn-on or pass resistances) of the switches used in the switched capacitor technique. Through the ON resistances of the switches used, a noise charge proportional to the factor k*T*C develops on the sensor capacitor, wherein k represents the Boltzmann constant, T the absolute temperature, and C the capacitance of the sensor. This sampled white noise in the sensor output signal will be referred to as so-called sampling noise in the following. Since the above capacitance value C is the overall capacitance of the capacitive sensor (sensor capacitor) and this overall capacitance often is substantially greater than the capacitance change, which generates the sensor output signal, by the measurement effect used by the sensor, this sampled white noise often leads to a limitation of the resolution of the measurement signal that can be reached in readouts in the switched capacitor technique.